A liftcrane is a type of heavy construction equipment characterized by an upward extending boom from which loads can be carried or otherwise handled by retractable cables.
The boom is attached to the upper works of the liftcrane. The upper works are usually rotatable upon the lower works of the liftcrane. If the liftcrane is mobile, the lower works may include a pair of crawlers (also referred to as tracks). The boom is raised or lowered by means of a cable(s) or cylinder(s) and the upper works also include a drum upon which the boom cable can be wound. Another drum (referred to as a hoist drum) is provided for cabling used to raise and lower a load from the boom. A second hoist drum (also referred to as the whip hoist drum) is usually included rearward from the first hoist drum. The whip hoist is used independently or in association with the first hoist. Different types of attachments for the cabling are used for lifting, clamshell, dragline and so on. Each of these combinations of drums, cables and attachments, such as the boom or clam shell are considered herein to be mechanical subsystems of the liftcrane. Additional mechanical subsystems may be included for operation of a gantry, the tracks, counterweights, stabilization, counterbalancing and swing (rotation of the upper works with respect to the lower works). Mechanical subsystems in addition to these may also be provided.
As part of the upper works, a cab is provided from which an operator can control the liftcrane. Numerous controls such as levers, handles, knobs, and switches are provided in the operator's cab by which the various mechanical subsystems of the liftcrane can be controlled. Use of the liftcrane requires a high level of skill and concentration on the part of the operator who must be able to simultaneously manipulate and coordinate the various mechanical systems to perform routine operations.
The two most common types of power systems for liftcranes are friction-clutch and hydraulic. In the former type, the various mechanical subsystems of the liftcrane connect by means of clutches that frictionally engage a drive shaft driven by the liftcrane engine. The friction-clutch liftcrane design is considered generally older than the hydraulic type of liftcrane design.
In hydraulic systems, an engine powers a hydraulic pump that in turn drives an actuator (such as a motor or cylinder) associated with each of the specific mechanical subsystems. Hoists actuated by hydraulic motors use brakes for parking. Cylinder actuated hoists use load holding valves as their parking mechanism. The actuators translate hydraulic pressure forces to mechanical forces thereby imparting movement to the mechanical subsystems of the liftcrane.
Hydraulic systems used on construction machinery may be divided into two types--open loop and closed loop. Most hydraulic liftcranes use primarily an open loop hydraulic system. In an open loop system, hydraulic fluid is pumped (under high pressure provided by the pump) to the actuator. After the hydraulic fluid is used in the actuator, it flows back (under low pressure) to a reservoir before it is recycled by the pump. The loop is considered "open" because the reservoir intervenes on the fluid return path from the actuator before it is recycled by the pump. Open loop systems control actuator speed by means of valves. Typically, the operator adjusts a valve to a setting to allow a portion of flow to the actuator, thereby controlling the actuator speed. The valve can be adjusted to supply flow to either side of the actuator thereby reversing actuator direction.
By contrast, in a closed loop system, return flow from an actuator goes directly back to the pump, i.e., the loop is considered "closed." Closed loop systems control speed and direction by changing the pump output.
Open loop systems have been generally favored over closed loop systems because of several factors. In an open loop system, a single pump can be made to power relatively independent, multiple mechanical subsystems by using valves to meter the available pump flow to the actuators. Also, cylinders, and other devices which store fluid, are easily operated since the pump does not rely directly on return flow for source fluid. Because a single pump usually operates several mechanical subsystems, it is easy to bring a large percentage of the liftcrane's pumping capability to bear on a single mechanical subsystem. Auxiliary mechanical subsystems can be easily added to the system.
However, open loop systems have serious shortcomings compared to closed loop systems, the most significant of which is a lack of efficiency. A liftcrane is often required to operate with one mechanical subsystem fully loaded and another mechanical subsystem unloaded yet with both turning at full speed, e.g., in operations such as clamshell, grapple, and level-luffing. An open loop system having a single pump must maintain pressure sufficient to drive the fully loaded mechanical subsystem. Consequently, flow to the unloaded mechanical subsystems wastes an amount of energy equal to the unloaded flow multiplied by the unrequired pressure.
Open loop systems also waste energy across the valves needed for acceptable operation. For example, the main control valves in a typical load sensing, open loop system (the most efficient type of open loop system for a liftcrane) dissipates energy equal to 300-400 PSI times the load flow. Counterbalance valves required for load holding typically waste energy equal to 500-2,000 PSI times the load flow.
As a result of the differences in efficiency noted above, a single pump open loop system requires considerably more horsepower to do the same work as a closed loop system. This additional horsepower could easily consume thousands of gallons of fuel annually. Moreover, all this wasted energy converts to heat. It is no surprise, therefore, that open loop systems require larger oil coolers than comparable closed loop systems.
Controllability can be another problem for open loop circuits. Since all the main control valves are presented with the same system pressure, the functions they control are subject to some degree of load interference, i.e., changes in pressure may cause unintended changes in actuator speed. Generally, open loop control valves are pressure compensated to minimize load interference. But none of these devices are perfect and speed changes of 25% with swings in system pressure are not atypical. This degree of speed change is disruptive to liftcrane operation and potentially dangerous.
To avoid having to use an extremely large pump, many open loop systems have devices which limit flow demand when multiple mechanical subsystems are engaged. Such devices, along with the required load sensing circuits and counterbalance valves mentioned above, are prone to instability. It can be very difficult to adjust these devices to work properly under all the varied operating conditions of a liftcrane.
An approach taken by some liftcranes manufactures with open loop systems to minimize the aforementioned problems is to use multi-pump open loop systems. This approach surrenders the main advantage that the open loop has over closed loop, i.e., the ability to power many functions with a single pump.
In summary, although most presently available liftcranes generally use open loop hydraulic systems, these are very inefficient and this inefficiency costs the manufacturers by requiring large engines and oil coolers and it costs the user in the form of high fuel bills. Moreover, another disadvantage is that open loop systems in general can have poor controllability under some operating conditions.
It is thus desirable to provide a closed loop system to overcome the disadvantages associated with open loop systems. Closed loop systems however, are not inherently suited for control of liftcrane hoists or raising devices or subsystems. The energy from a weight being lowered must be absorbed somehow by the hoist. On hydraulic machines, this is typically done with load holding valves which dissipate the energy to heat. Since the oil flow in closed loop systems does not return to a reservoir, it is very difficult to remove this heat from the oil. Consequently, load holding valves are not practical for use in closed loop systems.
Without holding valves, the control logic which must be used for a closed loop winch is considerably more complicated than what is typically used for the open loop equivalent. Because of this, the control scheme for a closed loop liftcrane hoist is best implemented in software running on a programmable controller.
Basic to this hoist control method is the use of feedback from pressure and motion sensors to maintain the proper direction and speed of the hoist. While such an approach generally produces very accurate and smooth hoist control, it is difficult to match the responsiveness of systems that don't use feedback.
It is therefore desirable to provide a hoist control system that: 1) allows use of the closed loop hydraulic system, 2) produces smooth and accurate control characteristics typical of feedback architectures, and 3) provides the responsiveness normally associated with systems that do not require feedback.